The present invention relates generally to the field of instruments for spectrally measuring and analyzing optical properties of samples. Such instruments are presently used in industrial and agricultural application for colorimetry and for quantitatively analyzing the constituents of samples. Additional applications for such instruments are being developed in the field of medicine in which samples are spectrally analyzed for diagnostic purposes.
Examples of agricultural applications presently in use are instruments which accurately determine the oil, protein and water content in grain or soybeans. The traditional analytical laboratory techniques, such as the Kjeldahl technique for measuring protein, are extremely accurate but require the services of a skilled chemist. The results, furthermore, are not immediately or readily available. Buyers of agricultural products have demonstrated an increasing interest in accurate and rapid determinations of the moisture, protein and oil percentages of the various produces purchased. The wheat export market, for example, has seen the widespread introduction of selling on the basis of guaranteed protein content. This competitive pressure has increased the requirement of the commodity handler, from the country elevator to the export terminal, to sort rapidly and accurately grains and other products by their content, where applicable.
Additional agricultural and food applications include measurement of constituents in dairy products, cereal, beverages, fruits, meats, etc.
In the industrial market, this type of measurement is successfully applied to the following areas:
the textile industry for measuring lubrication on yarn and for fiber finish in nylon, polyester, cotton and others; PA1 tobacco industry for measuring the percent of tar and nicotine; PA1 paper industry for the chemical analysis of paper including coatings, thickness and moisture; PA1 plastic industry for measurement of tapes and film thickness; PA1 gasohol and petroleum industry for composition determination; PA1 cosmetics and perfume industry for measurement of oils and other ingredients.
In the pharmaceutical industry, this instrument is applied for measurement and identification nondestructively of drug composition.
The need for versatile, yet low cost, advanced equipment, which combines and improves upon recent scientific findings in the field of nondestructive testing of products has greatly increased. For maximum usefulness of commodity handlers, such an instrument must not place high demands on the skillfulness of the operator or require a specialized knowledge of the scientific basis for the end result.
Recent developments have provided instruments which are able to satisfy some of the above requirements of commodity handlers. The optical analyzer disclosed by Issac J. Landa in U.S. Pat. No. 4,285,596 entitled "Holographic Diffraction Grating System for Rapid Scan Spectral Analysis" provides an optical system for rapid, accurate spectral analysis of the reflectivity and/or transmissivity of samples. A concave holographic diffraction grating oscillated at high speed is utilized to provide a rapid scanning of monochromatic light through a spectrum of wavelengths. The grating is positively driven at a very high speed (typically, ten scans per second) by a unique cam drive structure comprising identically shaped conjugate cams. The rapid scan by the grating enables reduction of noise error by averaging over a large number of cycles. The rapid scan also reduces measurement time, and thus prevents sample heating by excessive exposure of light energy. A filter wheel having dark segments for drift correction is rotated in the optical path in synchronism with the grating. Source optics is employed to shape optimally the light source for the particular application. The system optics further includes an arrangement of lenses, including cylindrical lenses, to obtain the best light source shape which results in maximum light throughput. Fiber optics are also employed and arranged to meet the optimum requirements of the system for light collection and transmission through portions of the optical system.
A related instrument is disclosed by Isaac J. Landa in U.S. Pat. No. 4,264,205, entitled "Rapid Scan Spectral Analysis System Utilizing Higher Order Spectral Reflections of Holographic Defraction Gratings", which is a continuation-in-part of the previously mentioned Landa patent. The disclosed optical system is similar to that shown in its parent application, but includes a filter wheel divided into two arcuate segments separated by opaque segments arranged approximately 180.degree. apart. One arcuate segment of the wheel transmits only first order light. The other arcuate segments transmits only second order light. Separate photodetectors are employed during infrared analysis of samples for detecting first order and second wavelength transmissions, and an electronic decoder apparatus is utilized for switching between detectors.
The discussions of the related art contained in the two Landa patents are incorporated by reference into this document.
The analyzers disclosed in the two Landa patents suffer from a number of disadvantages. First such optical analyzers are limited in the accuracy of their measurement by the particular drive mechanism employed for oscillating the diffraction grating. Specifically, the complex cam drive mechanism employed to provide a linear spectral scan is relatively costly and inaccurate. The cam drive mechanism is needed to control the variation in the velocity of the grating during each scan in order to obtain the desired linear spectral scan. The complex cam drive mechanism introduces error in the analysis because of the very tight tolerances required of the camming surfaces.
Another disadvantage is that the filter wheel employed for blocking the light to provide a dark offset value requires careful synchronization with the oscillating of the grating to ensure that the light is blocked at the appropriate time. This increases the likelihood of error as well as increasing the number of parts in the analyzer.
Still another disadvantage of the former analyzers is that the transmissivity and reflectivity of samples is measured by different detectors. This approach introduces an additional variable into the analysis of the samples, which necessarily compounds errors. The requirement of two sets of detectors also increases costs.